This invention relates to controlling the flow of current to windings used in rotating machinery, and more particularly to controlling the flow of current to superconducting windings.
Superconducting windings are being used in electrical machinery and rotating machines because of their low loss characteristics. While the superconducting windings are maintained at cryogenic temperatures, the power supplies used to drive the superconducting windings are typically maintained at ambient temperatures (300xc2x0 K.).
In the design of electrical machinery, incorporating high temperature superconducting (HTS) windings (i.e., motors, generators, magnets), the heat leak associated with the leads carrying current from the power supply at ambient temperatures to the cryogenically cooled windings is an overriding design factor which dictates the cost and thermal capacity of closed-cycle cryogenic cooling apparatus. These losses increase as the temperature difference between ambient and coil temperature increases. A number of approaches have been suggested to minimize the impact of heat leaks in such systems especially those in which the leads carry currents approaching several thousand amperes. Unfortunately, where vapor cooling of leads is not an option, these approaches introduce high voltages into the system or do not eliminate the need for a high current lead pair entering the cryogenic environment with attendant heat leaks. In cases where the superconducting coil is rotating with respect to a warm stator coil, the problem of heat leaks into the cryogenic environment becomes more critical due to the design constraints imposed by the thermal path impedance of a stationary cryocooler coupled indirectly to a rotating heat load or constraints on the size, weight, and thermal capacity of a rotating cryocooler.
There exist a number of large scale commercial and defense applications of HTS coils (e.g., magnet systems, generators and synchronous motor field windings) which require relatively constant magnetic fields, and in which ample time is available to ramp the coil current up to its initial desired value prior to regulated operation. In electrical machine systems incorporating HTS windings, the current in the HTS coil is subject to flux creep due to the finite losses in the HTS conductor. The dissipation due to this finite albeit small resistive loss requires that the current be restored periodically, i.e., xe2x80x9cpumpedxe2x80x9d via regulating circuitry back to its desired level. The energy input requirement is only that required to make up for the flux creep. Electronic circuits and mechanisms, which perform these functions, are referred to as xe2x80x9cflux pumpsxe2x80x9d.
The invention features an exciter assembly and approach for supplying power to a superconducting load, such as a superconducting field coil, disposed within a cryogenic region of a rotating machine. The exciter assembly provides an efficient and reliable approach for transferring the electrical power energy across a rotating interface.
In one aspect of the invention, the exciter assembly includes a transformer having a primary winding and a secondary winding, positioned in a rotational reference frame relative to the primary winding, and a rotatable enclosure including a wall having an intermediate core formed of a high permeability material. The intermediate core is positioned between the primary winding of the transformer and the secondary winding of the transformer. In essence, the intermediate core acts as a flux xe2x80x9cwindowxe2x80x9d or xe2x80x9cshuntxe2x80x9d between the primary winding and the secondary winding. This arrangement advantageously eliminates the need for current leads which transition from room temperature to cryogenically cool temperature and are used to provide electrical power from the exciter assembly to the superconducting load. This approach for supplying power to superconducting loads is particularly well suited for HTS superconducting rotating machines, such as those described in co-pending applications, Ser. No. 09/415,626, entitled xe2x80x9cSuperconducting Rotating Machinesxe2x80x9d, filed Oct. 12, 1999, and Ser. No. 09/481,484, entitled xe2x80x9cHTS Superconducting Rotating Machinexe2x80x9d, filed Jan. 11, 2000, both of which are incorporated by reference.
Embodiments of this aspect of the invention may include one or more of the following features.
The primary winding is in the form of a stationary disk and the secondary winding is in the form of a rotatable disk axially spaced from the stationary disk to form a gap therebetween, the wall of the rotatable enclosure disposed within the gap, at least one of the stationary disk and the rotatable disk is formed of radial laminations, the intermediate core is formed of radial laminations, the stationary disk and the rotatable disk are each formed of core segments, each core section on each of the stationary disk and rotational disk disposed in a radial direction and angularly spaced from another core section of the stationary disk and rotational disk, respectively, and the intermediate core is formed of core segments, each core segment on each of the stationary disk and rotational disk disposed in a radial direction and angularly spaced from the core segments of the intermediate core.
The load is a superconducting coil including high temperature superconductor. The primary winding is in the form of a stationary disk and the secondary winding is in the form of a rotatable disk axially spaced from the stationary disk to form a gap therebetween. In essence, the rotating disk and stationary disk provide a transformer for inducing AC voltage and current in the superconducting load. In one embodiment, the stationary disk and the rotatable disk are formed of radial laminations or other suitable materials.
The exciter assembly can further include a resistive load and a switch for allowing energy from the superconducting load to flow to the resistive load in the event of a detected fault.
In another aspect of the invention, a rotatable enclosure of the type surrounding a housing having an internal volume for supporting cryogenically-cooled components, the rotatable enclosure comprising a wall including a flux window formed of a high permeability material, the flux window positioned between a primary of a transformer disposed external to the rotatable enclosure and a secondary of the transformer disposed within the rotatable enclosure.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.